Transponders are known that are fitted with a non-volatile memory and structured in a plurality of words. One type of data is generally attributed to each word used. Some words may be reserved for the transponder manufacturers, others for a national distributor and others still for the end client or user. In order to protect either the content of some words, or the read access without authorisation, a data protection register is provided. This register is generally formed by a protective word having a plurality of bits corresponding to a plurality of words, access to which is locked individually. The logic circuit is arranged such that it is possible to lock access to any word by changing the logic state of the corresponding bit in the protective word. For several applications, locking is definitive, i.e. irreversible, particularly for security reasons. Thus, it is necessary to ensure that the content of the protective word can not be altered as regards the bits corresponding to the definitively locked words.
The conventional programmable word write method of an EEPROM memory generally includes a preliminary step of deleting the content of the word prior to writing the new content. Two situations can lead to a loss of the correct protection word content. First of all, an error during processing of the protection word content can occur such that the content after writing is erroneous. Secondly, the transponder power supply or the lock procedure of a given word may be interrupted just after the protection word content has been deleted and before the new consequently modified word has been written into the protection word. This second situation, which can occur by chance or deliberately, raises a serious memory data protection problem since all of the words of the memory have free access. It is an object of the present invention to solve the aforementioned data protection problem for a non-volatile memory.